Most of the alumina utilized for the making of metallic aluminum is made by the Bayer process which involves the digestion of bauxite in a caustic medium. Digestion results in a caustic aluminate liquor from which alumina hydrate is recovered by precipitation. Bauxites used in the Bayer process contain, besides alumina values, a significant quantity of iron and depending on the source of bauxite, a considerable portion of the iron content can be present in the form of goethite [FeO(OH)]. Processing of goethitic bauxites is difficult, since generally high temperatures (in excess of about 245.degree. C) and/or additives, such as lime, NaCl or Na.sub.2 SO.sub.4, are required to convert the goethite content to hematite or magnetite. This conversion is desirable for many reasons, for example, it allows recovery of alumina from the bauxite in increased yield, further the settling of the digestion residue, the so-called "red mud", is substantially improved.
The problem of goethite to hematite and/or magnetite conversion has been known for many years. For example, Hungarian researchers in the 1950's have published several papers relating to the treatment of goethitic bauxites. Thus, for example, in an article by Lanyi in the Yearbook of The Hungarian Research Institute for Non-Ferrous Metals, Volume 1, pp. 5-28, 1956, conversion of the goethite content of Hungarian bauxites to well-settling magnetite has been achieved by the use of a tube digester operating at temperatures at about 300.degree. C and higher. Conversion was ascertained by the brownish color of the mud produced and also by X-ray analysis.
In U.S. Pat. No. 3,737,514 (King), a process has been disclosed for the treatment of goethite and titanium containing aluminiferous ores resulting in increased alumina recovery from the ores. The process involves a two-stage digestion method in the presence of a calcium compound added in an amount at least sufficient to convert substantially all of the titanium content of the ore to calcium titanate. The first stage of the digestion is accomplished at about 280.degree. to 350.degree. C, while the second stage of the process proceeds at about 200.degree. to 255.degree. C.
In German Offenlegungsschrift No. 2,335,732 (published Jan. 31, 1974), a process is described for the conversion of the goethite content of gibbsitic bauxites to hematite and magnetite by digesting a caustic slurry of the bauxites at the temperatures between 220.degree.-280.degree. C in the presence of 1-6% by weight CaO and 0.5-12% by weight NaCl. The combination of CaO and NaCl, according to this reference, allows conversion of goethite to hematite at the reported temperatures without using a pretreatment.
German Offenlegungsschrift No. 2,431,857 (published Jan. 30, 1975) discloses the conversion of the goethite content of boehmitic and/or diasporic bauxites to hematite at 180.degree.-300.degree. C in the presence of a catalytically acting combination of 2-6% by weight CaO and a small quantity of sulfate ion, generally 1.0 to 14 g/l in the digestion mixture.
In U.S. Pat. No. 3,966,874 (Featherston et al.), a process is shown capable of converting the goethite content of boehmite and/or diaspore containing bauxites. The process of this reference employs a two-stage method, wherein in the first stage, a caustic slurry of the bauxite is pretreated at a temperature at least 200.degree. C, followed by addition of a calcium compound and digestion at a temperature of at least 225.degree. C, preferably at about 230.degree.-250.degree. C.
The extent of prior art dealing with the subject of goethite to hematite and/or magnetite conversion clearly indicates the importance of this area to produce Bayer process alumina. This becomes even more significant with the gradual decrease in the quality of available bauxites employed for the production of alumina. More and more bauxites of high goethite and diaspore content are being utilized and recovery of alumina values from these lower grade ores pose not only technical problems but also decreasing alumina yields due to the increasing alumina content occluded in the goethitic and/or diasporic lattice of the ore. The prior art processes, operating at temperatures at or below about 250.degree. C with or without goethite to hematite enhancing additives, are only capable of converting a fraction of the total goethite content of these low grade bauxites and consequently the yield of alumina recovered from these ores is far from the values desired from an economic point of view. On the other hand, digestion processes operating at temperatures in excess of about 280.degree. C with accompanying high pressures, such as 1000-1500 psi (68-102 atmospheres) can obtain the desired alumina yield from these low grade ores, but the equipment involved in the digestion at these process conditions, particularly when the large volumes encountered in the Bayer process are considered, require complex and expensive installations. For example, tube digesters of extensive length (in excess of about 1000 m), have been recommended for this purpose where digestion of the bauxite slurry takes place at about 300.degree. C under highly turbulent flow. Turbulence is a necessary requirement of this type of process, otherwise deposition of scale takes place on the inner walls of the long tube resulting in blockage, reduced flow and inadequate heat transfer. The turbulent flow in this system requires an excessively long and straight flow pattern. Also, the high rate of turbulence exerts a steady stress on the tube, which, in combination with the high pressures utilized, renders the equipment subject to frequent maintenance.
The use of conventional digestion design at these temperatures and pressures requires not only high input of heat energy but also a large number of high pressure digester vessels, flash tanks and liquor heaters capable of safely withstanding the pressures generated. Due to the large volumes to be treated under these conditions, the construction cost of digestion vessels of suitable size becomes prohibitive and economically, as well as technically, undesirable. Consequently, up until now, the Bayer process operator had to accept a compromise between alumina yield and process conditions, resulting in lower alumina recoveries in the existing equipment.
It has now been discovered that it is economically possible and technically feasible to obtain maximum alumina recoveries from goethitic bauxites at high temperatures and pressures, for example, in excess of about 250.degree. C. This is accomplished in a novel pretreatment system where bauxite slurries of high solids concentrations are brought to the goethite to hematite and/or magnetite transformation temperature without requiring complex and expensive equipment capable of accommodating the large volumes usually encountered in the Bayer process. Due to the treatment in this system of a highly concentrated bauxite slurry, the heat input required for the aforementioned transformation is significantly less than required either in the tube digesters or conventional digesters. The novel pretreatment system consists of at least one float-type, liquid displacement pressure pump employed to charge bauxite slurry of high solids content at high pressures into a heating device where the slurry is heated to the goethite transformation temperature or above. The heated slurry is then continuously cooled without flashing while still under high pressure, to the conventional Bayer process digestion temperatures in a suitable cooling means. This high pressure slurry is then charged to at least one float-type, liquid displacement pump where its pressure is reduced without flashing and is then charged by this pump to the conventional Bayer process digester. The use of the float-type liquid displacement pumps in combination with the heating and cooling means of the system allows high temperature and pressure pretreatment of the bauxite in a simplified, economically advantageous manner.